Remote plasma source for pre-treatment of substrates prior to deposition

ABSTRACT

A plasma processing chamber particularly useful for pre-treating low-k dielectric films and refractory metal films subject to oxidation prior to deposition of other layers. A remote plasma source (RPS) excites a processing gas into a plasma and delivers it through a supply tube to a manifold in back of a showerhead faceplate. The chamber is configured for oxidizing and reducing plasmas in the same or different processes when oxygen and hydrogen are selectively supplied to the RPS. The supply tube and showerhead may be formed of dielectric oxides which may be passivated by a water vapor plasma from the remote plasma source. In one novel process, a protective hydroxide coating is formed on refractory metals by alternating neutral plasmas of hydrogen and oxygen.

FIELD OF THE INVENTION

The invention relates generally to plasma processing of substrates suchas semiconductor integrated circuits. In particular, the inventionrelates to a plasma treatment chamber having a remote plasma source.

BACKGROUND ART

Plasma processing is widely used in the fabrication of semiconductorintegrated circuits including some of the fundamental processes ofplasma etching and deposition including plasma enhanced chemical vapordeposition (PECVD) and magnetron sputtering.

Vias are common structures used to vertically connect differentmetallization layers in integrated circuit. As illustrated in thecross-sectional view of FIG. 1, a lower dielectric layer 10 has aconductive feature 12 formed in its surface. An upper dielectric layer14 is deposited over the lower dielectric layer 10 and its conductivefeature 12 typically in a PECVD process. Photolithography including thepatterning of a photoresist layer etches a via hole 16 through the upperdielectric layer 14 down to the conductive feature 12. The via hole 16is filled with a metal to electrically connect the conductive feature 12to another layer of metal formed above the upper dielectric layer 14. Toprevent the metal from diffusing from the via 16 into the dielectriclayer 14, a thin conformal barrier layer is deposited onto at least thesidewalls of the via hole 16 prior to the metal filling. Typical barriermaterials are titanium for aluminum metallization and tantalum forcopper metallization along with nitrides of the barrier metal. Magnetronsputtering is often used for depositing both the barrier metal and itsnitride. This process for forming vertical interconnects may be repeatedseveral times for the increasing number of metallization layers requiredfor advanced logic circuits. In the lowest metallization level, the viais properly called a contact. In this case, a silicon layer replaces thelower dielectric layer 10 and the conductive feature may be a dopedregion in the silicon layer with accompanying contact layers such assilicides.

Many advanced integrated circuits replace the simple illustrated viawith a dual-damascene structure having a narrow via formed in the lowerportion of the dielectric layer and a connecting wider trench formed inthe upper portion. The trench may extend axially over significantdistances to provide horizontal interconnects in conjunction with thevertical interconnect of the via. The via and trench usually requireseparate etching steps but a single barrier deposition step and a singlemetal fill step are performed. The dual-damascene structure isparticularly advantageous for copper metallization since copper is notreadily etched. Instead, electrochemical plating (ECP) or relatedplating process fills the via and trench with copper and forms copperabove the dielectric layer 14. Chemical mechanical polishing (CMP)removes the excess copper outside of the trench. The copper-filledtrench may act as the conductive feature 12 in the lower metallizationlayer. Plating copper, however, requires a thin copper seed layerbetween the barrier layer to nucleate the copper formation and providinga plating electrode. Magnetron sputtering is most commonly used fordepositing the copper seed layer. Further discussions of the viastructure will apply equally well to dual-damascene structures, whichoften co-exist on the same chip.

In addition to the principal fabrication steps, plasma processing isalso used for plasma treating preexisting layers other than removingexposed layers as is done in etching. Plasma ashing performed after anetching process removes the patterned photoresist, typically composed ofa carbonaceous polymer. Often prior to a deposition step, plasmapre-cleaning removes photoresist residues 18 and other contaminantssettling on the via sidewalls wafer either because of prior processingor while the wafer is removed from the high-vacuum processingenvironment. Plasma pre-cleaning may also remove a native oxide layer 20that has developed on exposed metal layers. Surface residues 22 may alsoform above the lip of the via hole.

Plasma treatments have recently been beneficially applied to low-kdielectric materials used to as the inter-level dielectric layer 14 inorder to reduce capacitive coupling between narrowly spaced features andlayers. Advanced low-k dielectrics having dielectric constants of 3 andbelow may have a significant carbon content and may also be porous tofurther decrease the dielectric constant. When vias are etched throughdielectric layer to provide for vertical interconnects, the plasmaetching may damage the soft low-k dielectric material. Further, theporous dielectric material may be unsatisfactory as a base fordepositing subsequent barrier and metal layers. Accordingly, thepractice has developed of both plasma cleaning the damaged low-kdielectric and of stuffing oxygen into the pores of the etched low-kmaterial prior to the next deposition.

SUMMARY OF THE INVENTION

A plasma processing chamber may be configured to perform both reducingand oxidizing plasma processing, for example, pre-cleaning of an etchedsubstrate prior to metal deposition or treatment of a porous dielectriclayer exposed in the etching.

In one embodiment of the chamber, a remote plasma source is mounted onthe chamber and its output is supplied through a supply tube to a gasmanifold in back of the faceplate of a showerhead in opposition to apedestal supporting the wafer. An ion filter may be fitted to the supplytube. The supply tube and the manifold may be lined with an oxidedielectric such as alumina or quartz and the faceplate may be formed ofsuch materials. Sources of hydrogen gas, oxygen gas, and water vapor areconnected through respective metering valves to the input to the remoteplasma source.

The invention enables the formation of an hydroxide film by firstexposing the substrate to a neutral hydrogen plasma and then to aneutral oxygen plasma. The hydroxide film is particularly useful toprotect a refractory metal layer only partially covered by a thin copperseed layer and thus subject to local oxidation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a via showing the formation ofresidue and oxide.

FIG. 2 is a schematic cross-sectional view of plasma processing chamberusable with the invention.

FIG. 3 is a graph showing the formation of an oxidized refractory metal.

FIG. 4 is a cross-sectional view of a via illustrating the formation ofoxidized tantalum oxide which may be removed according to a use of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Many popular plasma treatments in the fabrication of semiconductorintegrated circuits may be divided into oxidizing treatments andreducing treatments. For plasma treatments, oxygen is the most commonlyused oxidizing agent when the chemical reactions of halogens need to beavoided and hydrogen is the most commonly used reducing agent. Onedifficulty of plasma treatments, particularly of porous low-k dielectricmaterial, is that a plasma generated adjacent the wafer, often called anin situ or local plasma, contains a high number of electrically chargedions, for example, of oxygen ions O⁺ or O₂ ⁻ or hydrogen ions H⁺. Even awafer left electrically floating tends to develop a significant negativeself-bias in the presence of a locally generated plasma. As a result,the positively charged ions within the plasma are accelerated towardsthe wafer and strike the wafer with high energy, often sufficient todamage the already deposited material and possibly sputter material fromone section of the developing integrated circuit to another.

For these reasons, the technology of a remote plasma source (RPS) hasbeen developed to generate a plasma of the treatment gas at a locationremote from the wafer, often outside of the main vacuum processingchamber. The plasma then diffuses into the processing chamber to treatthe wafer without the generation of a local plasma and the attendantnegative self-bias on the wafer. Furthermore, at least in the case ofhydrogen, ion filters may be placed between the remote plasma source andthe vacuum chamber to remove the hydrogen ions from the diffusing plasmaand leave only neutral hydrogen radicals, mostly H*. Radicals are highlyreactive but are not subject to electrostatic acceleration towards abiased wafer. They perform a soft etch or other treatment as opposed tothe hard etch or sputtering often resulting from heavy chargedparticles. However, attaching a remote plasma source to a vacuumprocessing chamber may present significant problems dependent upon thetreatment gas being excited into a plasma to process the wafer.

A cleaning chamber 30 illustrated in the cross-sectional view of FIG. 2includes a vacuum chamber 32 having an openable lid 34 and pumped by avacuum pump system 36. A pedestal 38 within the chamber 32 supports awafer 40 to be cleaned in opposition to a gas showerhead including ashowerhead faceplate 42 supplying a process gas into the vacuum chamber32 through a large number of apertures 44 formed in and distributed overthe faceplate 42. The pedestal 38 may include a resistive heater 46selectively supplied with current from a heater power supply 48 to raisethe temperature of the wafer 40 to a desired etching or pre-cleaningtemperature.

The process gas for one type of cleaning is either pure hydrogen gas(H₂), which is supplied from a hydrogen gas source 50 through a massflow controller 52, or a combination of hydrogen and helium (He), whichis supplied from a helium gas source 54 through another mass flowcontroller 56. A remote plasma source 60 receives the gas and excites itinto a plasma. The remote plasma source 60 may be a capacitive exciterhaving a pair of electrodes positioned on opposed sides of a deliverytube delivering the process gas and driven by an RF power source or aninductive exciter having an inductive coil wrapped around the deliverytube and driven by the RF power source. Other types of plasma generatorsare possible. The excited gas or plasma is delivered though a supplytube 61 to a gas manifold 62 in back of the showerhead faceplate 42. Theexcited gas is thus delivered uniformly through the showerhead faceplate42 to the wafer 40 being cleaned.

The remote plasma source 60 is located upstream of the vacuum chamber32. An ion filter is disposed along the path between the remote plasmasource 60 and the manifold 62 to remove any hydrogen ions H⁺ so thatonly neutral hydrogen radicals H* of the plasma reach the wafer 40. Theion filter may include two magnets 63, 64 disposed in opposition acrossthe supply tube 61 to project a magnetic field B across the tube'sinterior to deflect or capture the charged hydrogen ions. Since theplasma generating unit of the remote plasma source 60 is not completelyefficient, neutral but unexcited hydrogen molecules H₂ also reach thewafer 40 but they are much less efficient than the hydrogen radicals ina reducing reaction. Other forms of ion filters may be used.

A removable dielectric tube liner 66 may be placed inside the supplytube 61 and a dielectric chamber liner 68 may cover the walls of themanifold 62 to protect them and to reduce recombination of the hydrogenradicals. The liners 66, 68 are typically oxide ceramics. For example,the tube liner 66 may be composed of alumina (Al₂O₃) and the manifoldliner 68 and the showerhead faceplate 42 may be composed of quartz(SiO₂). However, the reducing hydrogen chemistry may also etch the oxideliners 66, 68 and showerhead faceplate 42.

Water passivation of the liners 66, 68 and showerhead faceplate 42 isdescribed by Fu in patent application Ser. No. 11/351,676, filed Feb.10, 2006, now published as patent application publication 2007/0190266,and incorporated herein by reference. A vacuum-sealed ampoule 70containing a pool 72 of liquid water is mounted on the chamber lid 34and a mass flow controller 74 meters water vapor from the ampoule 70into the remote plasma source 60. The vapor pressure of water at roomtemperature is about 20 Torr, which is well above the usual vacuumlevels at which the remote plasma source 60 operates. Accordingly, oncethe ampoule 70 has been back pumped, a water vapor having a pressure ofabout 20 Torr exists in a head space 76 above the liquid water pool 72in the ampoule 70. The ampoule 70 is mounted directly on the chamber lid34 to minimize the length of tubing on the walls of which water vapor islikely to condense while the gas sources 50, 54 and their mass flowcontrollers 52, 56 are typically mounted on a remote gas panel withsomewhat long tubing 78 to the chamber 30 and its remote plasma source60.

A controller 80 controls the operation of the variable elements of thecleaning chamber 30 such as the vacuum pump system 36, the mass flowcontrollers 52, 56, 74, and the remote plasma source 60. The control isperformed according to a process recipe for the desired application. Therecipe is recorded in a recordable medium 82, such as a CDROM, insertedinto the controller 80.

In one method of using the chamber 30, the water vapor is not used inplasma cleaning the wafer 40. Instead, water vapor is pulsed into thechamber vacuum chamber 32 before the wafer is heated, before the remoteplasma source 60 is turned on, and before the excited hydrogen issupplied to the vacuum chamber 32. The flow of water vapor isdisconnected before the excited hydrogen is applied to the heated wafer.Water vapor has been found to deleteriously affect some types of porouslow-k dielectric material so its exposure should be minimized in thesecases. The small amount of water passivates the walls of the chamber andparticularly the liners 66, 68 and showerhead faceplate 42 and protectthem from the reducing chemistry of the hydrogen clean. Further, thepassivation both lengthens the lifetime of the remote plasma source 60and decreases the generation of particles, presumably from these walls.It also increases the performance of the pre-cleaning as measured by thephotoresist etch rate and selectivity between the photoresist and low-kdielectric.

The same chamber can be adapted to deliver a remotely generatedoxidizing plasma. An oxygen gas source 90 supplies oxygen gas through amass flow controller 92 to the remote plasma source 60, which excitesthe oxygen into a plasma. The ion filter comprising the magnets 62, 64removes the oxygen ions such as O⁺ from the plasma as it diffuses withinthe tube liner 66 towards the manifold 56 of the showerhead faceplate42. Thereby, principally neutral oxygen radicals O* are delivered as theexcited oxidizing species into the interior of the vacuum chamber 32 totreat the wafer 40 to a soft plasma oxidization. Alternative neutraloxygen radicals include ozone O₃ and excited atomic states of O₂*. Theplasma generator 60 is not completely efficient so neutral unexcited O₂molecules reach the wafer but they are less effective at oxidizing thanthe oxygen radicals. The oxide liners 66, 66 and showerhead faceplate 42are well adapted to the oxidizing plasma and do not usually requirewater passivation.

The so configured cleaning chamber 30 can be used in a number of diverseapplications, some of which are described below in more detail. However,the operation of the chamber is not limited to the described processembodiments.

Residue Cleaning

The chamber of FIG. 2 may be used in a pre-cleaning step prior tobarrier deposition in which residue is removed by a hydrogen plasma. Fuet al. have described hydrogen pre-cleaning process using a remoteplasma source in U.S. patent application Ser. No. 11/334,803, filed Jan.17, 2006, now published as application publication 2007/0117397, andincorporated herein by reference. Fu has described wall passivation usedin a hydrogen pre-clean with such a chamber in aforementioned patentapplication Ser. No. 11/351,676.

Hydrogen Ashing

Yang et al. have described in U.S. patent application Ser. No.11/737,731, filed Apr. 19, 2007, now published as applicationpublication 2008/0261405, and incorporated herein by reference, aprocess that may be practiced in the chamber of FIG. 2 or an extensionof it. Their process includes an initial pre-treatment of a low-kdielectric with a plasma of hydrogen and nitrogen and a main ashing stepfor removing photoresist with a plasma of hydrogen, water vapor, argon,and optionally methane. We believe the water vapor may introducedifficulties with the low-k dielectric.

Oxygen Stuffing of Metal Barriers

The combined cleaning chamber 30 described above can apply neutraloxygen radicals to a wafer pre-formed with a titanium barrier layer onthe via sidewalls before aluminum is filled into the via. This operationis sometimes referred to as oxygen stuffing. Titanium films typicallyform as a polycrystalline metal having grain boundaries between thecrystallites. The grain boundaries provide a diffusion path for aluminumatoms from the metallization to the underlying dielectric particularlywhen the titanium barrier layer is thin. When oxygen is stuffed into thegrain boundaries, it blocks the diffusion path.

Similar types of oxygen stuffing may be applied to barriers of otherrefractory metals, such as tantalum, ruthenium, an alloy of rutheniumand tantalum, and tungsten.

This method contrasts with previously used methods of diffusing oxygengas O₂ under elevated temperatures into the titanium or of depositing anoxygen-containing layer over the titanium layer and overcoating it witha titanium nitride layer. Both methods suffer from the weak bonding ofthe oxygen with the matrix of the titanium thin film. Yet another methodincludes ion implanting oxygen into the titanium film from a local. Thismethod suffers from the relatively high energy of the incident oxygenions, which tends to break bonds in the thin film and generatecrystalline defects, such as dislocations, voids, and interstitials.

Oxygen Stuffing of Nitride Barriers

The combined cleaning chamber 30 can also apply neutral oxygen radicalsto a wafer pre-formed with a nitride barrier layer, such as titaniumnitride (TiN) and tantalum nitride (TaN). Atomic oxygen is highlyreactive with the nitride and causes a stable barrier to immediatelyform.

Experiments have shown that TiN barriers treated with neutral oxygenradicals from a inductively coupled remote plasma source provides aneffective barrier in a Cu-barrier-Al stack with half the thickness of aTiN barrier thermally annealed in an O₂ ambient. Further, thelow-temperature thermally activated neutral plasma process imposes lessof a heat load on the integrated circuit, increases the throughput, andproduces no direct ion bombardment that potentially causes otherproblems.

Gases such as hydrogen (H₂), nitrogen (N₂) and water vapor may be addedto the remote plasma source to modulate the recombination rate of atomicoxygen back to O₂.

Tungsten Oxide Reduction

Sometimes a layer of tungsten oxide (WO) needs to be removed from adeveloping integrated circuit. For example, after CMP has removed theexcess copper or aluminum in a via and before the next level of metal isdeposited, tungsten oxide needs to be removed. The tungsten oxide may beremoved by a remotely generated hydrogen plasma supplying mostlyhydrogen radicals as the active species to the wafer.

The process was verified with a test sample including a 10 nm-thick filmof tungsten which was thermally oxidized in air. The reflectivity andsheet resistance was measured for the oxidized but uncleaned sample withthe results shown in the graph of FIG. 3. The sample was thenpre-cleaned in the reducing environment of neutral hydrogen radicals.After 25 s of treatment, the reflectivity showed a significant increaseand the sheet resistivity a significant decrease. A further pre-clean of125 s showed further changes but not sufficient to justify the addedtime. That is, 25 s of reducing pre-clean is operationally sufficient inmost circumstances.

In another test, a 50 nm-thick tungsten layer was sputtered onto asilica substrate and oxidized at high temperature in an oxygen plasma.Two samples were alternatively pre-cleaned or not. With or withoutpre-cleaning, the two samples were capped with 20 nm of sputteredtantalum. SIMS analysis shows a distinct oxygen peak between thetantalum and metallic tungsten if no pre-cleaning is performed betweenthe oxidation and tantalum coating. With 25 s of pre-cleaning, the oxidepeak is reduced by a factor of ten and there is a sharper interfacebetween the tungsten and tantalum.

Hydroxide Film

The cleaning chamber 30 of FIG. 2 can be used to form a protectivehydroxide layer on a metal, particularly a refractory metal. Forexample, as illustrated in the cross-sectional view of FIG. 4 a coppermetallization is often formed by first sputtering a barrier layer 100 ofa refractory metal such as tantalum onto sidewalls of the via 16although preferably not its bottom and then sputtering a thin layercopper seed layer 102 over the barrier layer 100. However the copperneeds to be thin within the narrow via 16 and is likely to leaveopenings 104 exposing the tantalum or other refractory metal of thebarrier layer 100. The exposed refractory metal is likely to oxidize toa refractory oxide such as tantalum oxide when the wafer is transferredto the plating bath. The tantalum oxide is less conductive than tantalumand thus increases the resistance of the plating electrode in ECP.Further, tantalum oxide more poorly initiates copper growth thanmetallic tantalum.

The hydroxide film may be formed in a two-step process upon a wafer 40placed into the chamber 30 and containing an exposed metal layer. First,oxygen is metered from the oxygen source 90 into the remote plasmasource 60 to supply neutral oxygen radicals into the chamber 32.Although the precise mechanisms are not completely understood, it isbelieved that the neutral oxygen radicals oxidize a thin surface layerof the refractory metal or at least are adsorbed to the metal atoms onthe surface. Because the refractory metal is resistant to oxidation, theoxidation is limited to near the surface. Secondly, the flow of oxygenis stopped and hydrogen is metered from the hydrogen source 50 throughthe remote plasma source 60 into the vacuum chamber 32. The neutralhydrogen radicals act to partially reduce the metal oxide or at leastbond with the adsorbed oxygen. However, rather than completely reactingwith the oxygen to form water, the reaction produces hydroxyls —OH,which bond to the surface of the metal film and thereby form an hydroxylor metal hydroxide film.

The hydroxide film is formed under controlled high-purity conditions andthus produces a more reliable protective film than one formed byexposure to ambient air and its water vapor.

The hydroxide film acts as a barrier to protect the metal film fromextensive oxidation when the wafer is exposed to air. Further,hydroxides tend to dissolve more readily in acid electrolytes than dooxides. Accordingly, the hydroxide film is effective at allowing thequick recovery of the conductivity of thin metal films when the wafer isimmersed in the electrolytic bath for processes such as copper ECP.

Although it is possible to first expose the wafer to the neutralhydrogen plasma and then the neutral oxygen plasma or to simultaneouslysupply both oxygen and hydrogen to the remote plasma source, it isbelieved that the hydroxyl formation is promoted by first oxidizing andthen reducing.

The combined chamber is capable of being applied to a number ofdisparate applications, some of which are themselves novel, withoutmajor hardware changes. Furthermore, the combined chamber allows the useof both oxygen and hydrogen radicals in novel applications, such as thedescribed hydroxide film.

1. A plasma processing chamber, comprising: a vacuum chamber; a pedestaldisposed in the vacuum chamber for supporting a substrate to beprocessed; a showerhead in opposition to the pedestal and including agas manifold and a plurality of apertures through a face plate andbetween an interior of the gas manifold and an interior of the vacuumchamber; a plasma generating unit; an inlet tube connected between anoutput of the plasma generating unit and the manifold; an ion filteroperatively associated with the inlet tube, including magnets projectinga magnetic field across an interior of the inlet tube, and operating toremove ions from a plasma passing from outlet of the plasma generatingunit through the inlet tube and to pass neutral radicals in the plasma;a source of oxygen gas selectively connectable to the plasma generatingunit; a source of water vapor comprising an ampoule containing watermounted on the vacuum chamber and selectively connectable to the plasmagenerating unit; and a source of hydrogen gas selectively connectable tothe plasma generating unit.
 2. The chamber of claim 1, furthercomprising a source of helium gas selectively connectable to the plasmagenerating unit.
 3. The chamber of claim 1, further comprising controlmeans for executing a process recipe for plasma treating a substrateincluding activating the plasma generating unit while concurrentlyconnecting both the source of oxygen gas and the source of hydrogen gasto the plasma generating unit.
 4. The chamber of claim 1, furthercomprising control means for executing a process recipe for plasmatreating a substrate including activating the plasma generating unitwhile alternatingly connecting the source of oxygen gas and connectingthe source of hydrogen gas to the plasma generating unit while thesubstrate is disposed in the vacuum chamber.
 5. The chamber of claim 1,further comprising a removable liner consisting of an oxide material andplaced inside the inlet tube.
 6. The chamber of claim 1, wherein theampoule is back pumped to reduce a pressure in a head space above a poolof the water within the ampoule.
 7. The chamber of claim 1, wherein awall surface of the manifold other than the face plate comprises anoxide material.
 8. The chamber of claim 7, wherein the wall surface isincorporated into a liner placed on a side of a wall of the manifoldfacing the interior of the manifold.
 9. The chamber of claim 7, whereinthe oxide material comprises quartz.
 10. The chamber of claim 9, whereinthe faceplate comprises quartz.
 11. The chamber of claim 9, furthercomprising an alumina liner placed inside the inlet tube.
 12. A processfor processing a substrate disposed in the vacuum processing chamber ofclaim 1, comprising the steps of: a first step of remotely generating inthe plasma generating unit a first plasma of oxygen and admitting it tothe chamber; and a second step of remotely generating in the plasmagenerating unit a second plasma of hydrogen and admitting it to thechamber.
 13. The process of claim 12, wherein the first and second stepsare performed sequentially.
 14. The process of claim 12, wherein thefirst and second steps are performed concurrently.
 15. The process ofclaim 12, wherein the substrate contains an exposed layer of a metal.16. The process of claim 15, wherein the metal comprises tantalum. 17.The process of claim 16, wherein the layer of a metal is partiallyoverlaid by a copper layer.
 18. The process of claim 12, wherein thesubstrate comprises: a dielectric layer with a hole formed therein; abarrier layer comprising tantalum formed on sidewalls of the hole; and aseed layer comprising copper formed over the barrier layer on thesidewalls.
 19. The process of claim 12, wherein the process furthercomprises prior to the first and second steps and prior to turning onthe plasma generating unit admitting water vapor from the source ofwater vapor from the source of water vapor to the plasma generatingunit.